1. Field of the Invention
This invention relates to indicators for time and temperature and methods for their preparation. More particularly, it relates to cholesteric liquid crystals capable of incorporation into indicator devices suitable for detecting limit-temperatures and/or limit-times. In addition methods are disclosed for reliably producing repeat batches of cholesteric liquid crystals.
2. Description of the Prior Art
Many physiologically active substances, such as blood and its components, biologicals, and blood replacements such as polymeric synthetic blood, degrade rapidly under ambient temperatures and must be stored under refrigeration or frozen prior to being used to minimize degradation. Warming to a temperature above a level predetermined as "safe" for each substance may cause degradation sufficient to prohibit the intended use of the substance. Such warming can occur if the cooling apparatus malfunctions, during shipment, or after the substance is removed from refrigeration for use.
Physiologically active substances, especially blood, are used in large quantities. They are commonly either frozen or refrigerated at about 1.degree.-8.degree. C. After they are removed from regulated cold storage, as in a blood bank, they may be transported to other locations for use, or possible use. The time in transit, in temporary refrigerated storage, or at room temperature, frequently extends for a number of hours before they are used or returned to regulated storage. One example of this storage problem is blood storage and handling. In order to safeguard against degradation of blood contained in medical blood bags, handling procedures are set up, which are to be followed by hospital personnel, and records of the time/transport of the transitory history of the blood bags are maintained.
There has been no means or method for continuously monitoring the temperature of blood and other physiologically active substances on a large scale, in such a manner as to record the occurrence of a temperature beyond acceptable limits and make the information known upon inspection at a later time. As a result, many substances are discarded after they have been removed from cold storage for a short period of time. Consequently, a great need exists for such means and/or method, which will insure that an acceptable temperature limit has not been exceeded, though a substance be removed from reliable refrigeration for an extended period of time. If it is known that the substance has not exceeded an acceptable temperature, in many cases the substance can be restocked in cold storage to be reissued (i.e., whole blood) and thus avoid disposing of expensive physiologically active substances.
Glass thermometers have been used to monitor the temperature of refrigerated physiologically active substances. To avoid contamination, direct contact between the thermometers and the substances should be avoided. Consequently, a thermometer typically is sandwiched between containers of a substance in transit. However, the temperature indicated by the thermometer fluctuates with the temperature of the substance, and the thermometer does not preserve an indication of the highest temperature reached. Consequently, the thermometer must be observed at relatively frequent intervals in order to provide some assurance that the substance has not exceeded its predetermined safe temperature.
Special glass thermometers are available having a "memory" feature: these thermometers after first being adjusted or set will indicate the highest temperature encountered by the thermometer after its adjustment or setting. The high reading is semipermanent, i.e., it is maintained until the thermometer is readjusted or reset. These special thermometers thus can reveal that the temperature of a monitored substance has exceeded a predetermined temperature even though the substance may be recooled after having exceeded the predetermined temperature. However, glass thermometers of either type are expensive, the special type being the more expensive. In addition, glass thermometers are relatively susceptible to breakage, giving rise to the hazards of broken glass including rupture and contamination of substances stored in thin plastic bags and, if the thermometer contains mercury, of toxic pollution.
Thermometric devices employing cholesteric liquid crystalline materials, referred to herein at times simply as "liquid crystals," heretofore have been employed for monitoring the temperature of stored physiologically active substances. These devices are inexpensive and not susceptible to breakage, but they do not have a memory feature. Therefore, these devices, like ordinary glass thermometers, must be observed at relatively frequent intervals to provide assurance that the substance has not exceeded its safe temperature.
Certain cholesteric liquid crystalline materials can be employed in memory-type thermometeric devices. These liquid crystals are of the type, disclosed in U.S. Pat. No. 3,594,126, which can form a metastable ordered glass under the appropriate conditions. The liquid crystal thermometeric devices can be manufactured to give an indication of whether a predetermined temperature or predetermined time at a specific temperature has been exceeded, and will maintain this indication until reset. The use of metastable ordered glass-forming liquid crystals to indicate thawing and exceeding of predetermined temperatures is disclosed in the foregoing patent and in U.S. Pat. No. 4,161,557. However, there was no appreciation or awareness of the pseudo-irreversible phase of the metastable ordered glass state of cholesteric liquid crystals, said phase being capable of monitoring multiple predetermined temperatures (limit-temperatures), all of such limit-temperatures being above the glass-transition temperature minimum. In fact, the '126 patent taught away from this discovery (column 2, lines 15-33). As far as is known to applicants, no such composition has been provided heretofore which has solved the problem of reliably producing a cholesteric liquid crystal capable of insuring that a physiologically active substance has not exceeded an acceptable temperature limit though removed from reliable refrigeration for an extended period of time.